1. Field of the Invention
The present invention relates to a fuel pump control system for an internal combustion engine. More specifically, the invention relates to a fuel pump control system for controlling the capacity of a positive-displacement type fuel pump driven by a camshaft of an internal combustion engine.
2. Description of the Related Art
A common rail type fuel injection system is known in the art. The common rail type fuel injection system includes a common rail (a reservoir) for receiving high pressure fuel and fuel injection valves connected to the common rail for injecting fuel into the cylinders of the engine. Since the injection rate of the fuel injection valves changes in accordance with the fuel pressure in the common rail, the pressure of the fuel in the common rail must be precisely controlled based on the operating condition of the engine in order to achieve a fuel injection rate suitable for the engine operating condition.
The common rail pressure (the pressure of the fuel in the common rail), in general, is controlled by adjusting a discharge capacity (fuel feed amount) of a high pressure fuel supply pump supplying high pressure fuel to the common rail. As a high pressure fuel supply pump, usually a positive-displacement type pump such as a plunger pump driven by a driving cam coupled to the camshaft of the engine and rotating synchronously with the camshaft, is used.
A control system of the fuel pump of this type is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 8-177592. The control system in the '592 publication utilizes a plunger pump driven by a driving cam being coupled to the engine camshaft and rotates synchronously therewith. The control system in the '592 publication determines a target common rail pressure based on the operating condition of the engine and controls the discharge capacity of the plunger pump in accordance with the target common rail pressure and an actually detected common rail pressure so that the actually detected common rail pressure coincides with the target common rail pressure. The detection of the actual common rail pressure and the discharge of fuel from the pump are performed at every predetermined rotation angle of the crankshaft of the engine.
However, problems may occur if the system in the '592 publication is applied to an internal combustion engine equipped with a variable valve timing device for adjusting the valve timing of the engine in accordance with the engine operating conditions.
In a certain type of variable valve timing device, the valve timing of the engine is adjusted by changing the rotational phase of the camshaft relative to the crankshaft. If the fuel pump control system in the '592 publication is applied to an engine equipped with a variable valve timing device of this type, it becomes difficult to control the discharge capacity of pump, and the common rail pressure cannot be accurately controlled to the target pressure.
Namely, the plunger of the fuel pump in the '592 publication is moved by a driving cam to reciprocate within the cylinder of the pump. The driving cam of the pump is coupled to the camshaft of the engine and rotates synchronously with the camshaft. Therefore, the rotational phase of the driving cam also changes when the rotational phase of the camshaft is changed by the variable valve timing device.
Further, in the system of the '592 publication, discharge of the fuel from the pump is started when the angular position of the crankshaft reaches a predetermined crank rotation angle and the discharge of the fuel continues until the end of the discharge period determined based on a target discharge amount. the discharge period is given by an angle of rotation of the crankshaft. The discharge capacity of the pump is determined by an amount of the effective discharge stroke (i.e., the displacement of the plunger during the discharge period) and, in other words, the amount of change in the cam-lift of the driving cam during the discharge period.
As explained above, since the driving cam of the pump rotates synchronously with the camshaft of the engine, the rotational phase of the driving cam changes when the rotational phase of the camshaft changes. Therefore, when both the crank shaft rotation angle (crank angle) at which the discharge period starts and the crank angle at which the discharge period ends are fixed, the discharge capacity of the pump also changes when the rotational phase of the driving cam relative to the crankshaft changes. This causes a change in the discharge capacity of the pump. Therefore, in the system of the '592 publication, the discharge capacity of the fuel supply pump changes when the valve timing of the engine changes if the discharge period of the pump is fixed.
This problem is illustrated in detail in FIG. 5.
In FIG. 5, the vertical axis represents the cam-lift of the driving cam of the pump and the horizontal axis represents the crank angle. The curve I in FIG. 5 shows the change in the cam-lift of the driving cam when the rotational phase of the camshaft is set to a value where the valve timing of the engine is most retarded, and the curve II shows the same when the rotational phase of the camshaft is set to a value where the valve timing of the engine is most advanced. As can be seen from FIG. 5, the cam-lift curve of the driving cam moves in the direction in which the crank angle advances when the valve timing of the engine advances. In this case, if the start and the end of the discharge period (expressed by crank angles) are fixed, i.e., if the discharge period is fixed at DP1 in FIG. 5, the effective discharge stroke D1 of the pump plunger when the valve timing is most retarded changes to D2 when the valve timing is most advanced. This means that the discharge capacity of the pump cannot be controlled precisely when the valve timing of the engine changes if the pump discharge is controlled, i.e., if the start and end of the discharge period are set in the manner same as that in the fixed valve timing engine. In this case, since an excess or a shortage of the fuel discharge capacity relative to the target discharge capacity occurs, problems such as a deviation in the actual common rail pressure, from the target value, and an increase in the engine power loss due to excessive work of the fuel pump may occur.